Electrostatic printing system and method using ions and toner particles

ABSTRACT

A system for electrostatic printing including a corona source and electrode for generating a substantially uniform ion stream in the direction of a print receiving medium and a multilayered apertured grid modulator interposed in the ion stream for modulating the cross sectional flow density of ions in the stream in accordance with a pattern to be reproduced. A toner supply introduces a cloud of substantially uncharged toner marking particles adjacent the print receiving medium whereby the modulated ion stream selectively impinges upon and charges toner particles in the cloud which are thereby accelerated and deposited on the print receiving medium in accordance with the pattern to be reproduced.

United States Patent Pressman et al.

[ Dec. 18,1973

ELECTROSTATIC PRINTING SYSTEM AND METHOD USING IONS AND TONER PARTICLESInventors: Gerald L. Pressman, Cupertino;

I-Iugh Frohbach, Sunnyvale; David E. Blake, Woodside, all of Calif.

Assignee: Electroprint, Inc., Palo Alto, Calif.

Filed: Dec. 28, 1970 Appl. No.: 101,681

US. Cl ..101/426,10l/1, 101/114, 101/D1G. l3, 355/3 Int. Cl G03g 13/06Field of Search lOl/DlG. 13, 426, 101/114, 1, 129; 355/3 ReferencesCited UNlTED STATES PATENTS Primary ExaminerEdgar S. BurrAttorney-Townsend and Townsend [5 7] ABSTRACT A system for electrostaticprinting including a corona source and electrode for generating asubstantially uniform ion stream in the direction of a print receivingmedium and a multilayered apertured grid modulator interposed in the ionstream for modulating the cross sectional flow density of ions in thestream in accordance with a pattern to be reproduced. A toner supplyintroduces a cloud of substantially uncharged toner marking particlesadjacent the print receiving medium whereby the modulated ion streamselectively impinges upon and charges toner particles in the cloud whichare thereby accelerated and deposited on the print receiving medium inaccordance with the pattern to be reproduced.

29 Claims, 19 Drawing Figures PAIENIEDUH: 1 8 1975 SREH 10$ 4 FIG..1A

l5 eeeef WWW.

FlG 2 INVENTORB GERALD L. PRESSMAN HUGH F FROHBACH DAVID E. BLAKE T 047%ATTORNEYS j 40 F IG 6 PATENTEUUEC 18 I975 SHEEIZBF 4 W1 m 4 h a 3% if QF t1; 1 w

WMIHLW 5 fi mw wdw R wi AMA/HM wi T 5 m w v.

V 1 J MW INVENTORS GERALD L. PRESSMAN HUGH F FROHBACH DAVID E. BLAKEATTORNEYS TONER FlG 8A 'TONER SUPPLY INVENTORS GERALD L. PRESSMAN HUGH EFROHBACH DAVID E. BLAKE PMENIEflum 1 8 ms 3,779,166 saw u or 4 7 a [y//X DRIVE 95 SCREEN DRIVE SCREEN DRIVE U I04 U I05 U I06 n2 V/ ////75,ll3

SCREEN omvE INVENTORS GERALD L. PRESSMAN g I HUGH F. FROHBACH TONERDAVID E. BLAKE FIG 16 7W 7mm ATTORNEYS ELECTROSTATIC PRINTING SYSTEM ANDMETHOD USING IONS AND TONER PARTICLES This invention relates to a newand improved system for electrostatic reproduction and in particular toan improvement in modulated aperture non-contact electrostatic printing,useful in a wide range of applications including document copiers,computer printouts, silent typewriters, etc.

The system of electrostatic printing referred to herein as modulatedaperture non-contact electrostatic reproduction is generally set forthin U.S. Pat. applications Ser. Nos. 673,499, now U. S; Pat. No.3,625,604

and 776,146 now U. S. Pat. No. 3,647,291 assigned to the assignee of thepresent case. According to these disclosures, there is provided in apreferred embodiment a multilayered apertured screen including atleast'a conductive layer and an adjacent insulative layer on which anelectrostatic latent image is formed for modulatinga flow of chargedtoner particles, ions or other printing particles directed through theapertures of the screen by an accelerating field. The minimum of twolayers for the screen construction permits establishing a double layercharge on the opposite sides of the insulative layer for selectivelyproducing overlapping lines of force or fringing fields within theapertures of the screen. Thus, lines of force generated by bi-polarelectrostatic fields extend within the apertures of the screen and canbe oriented to oppose the passage of charged particles, enhance the flowof charged particles, or be neutralized to have no effect on chargedparticles directed through the apertures of the screen. Thus, the doublelayer charge can be selectively established across the face of thescreen to substantially completely block the passage of chargedparticles through certain apertures, enhance and accelerate the passageof charged particles through other apertures, and control the width anddensity of the particle stream through other apertures over a continuousspectrum. A stream or flow of charged particles directed through thescreen by an overall applied accelerating field is therefore modulatedto provide a cross sectional density variation at least substantiallycorresponding with the image or pattern to be reproduced. The modulatedstream of toner particles or other charged particles passing through theapertured screen is transferred by the overall applied electrostaticprojection field across a gap or space to the print receiving medium.The pattern image formed by this non-contact printing arrangement isthereafter fixed according to known techniques if dry particles areused. If the toner particles consist of liquid aerosols the fixing stepmay not be necessary. Furthermore, if the charged particle streamconsists of ions the ions impinge on the paper forming an electrostaticlatent image which is thereafter dusted and fixed. In this latterembodiment, treated dielectric paper must be used in order to retain theelectrostatic. latent image formed by the ion stream.

In one approach for establishing the double layer charge or bi-polarelectrostatic latent image across the screen, a substantially uniformcharged distribution is initially established across both sides of theinsulative layer with opposite charges on the respective sides toprovide fringing fields within the apertures. These fringing fields areoriented to produce blocking fields to block or partially blockparticles of a predetermined charge sign. By use of photosensitiveinsulating materials the charge distribution and therefore the blockingfields are dissipated according to a light pattern projected on thescreen. The fringing fields also may be oriented to include enhancingfields which are established by additional techniques. The screen mustgenerally be charged initially with blocking fields which block thepassage of charged toner particles through the screen however. Theblocking fields are thereafter selectively neutralized according to thelight pattern or reversed to enhancing fields according to additionalprocedures, and the resulting modulation of the flow of toner particlesthrough the screen results in certain embodiments in negative printing.In order to achieve positive printing special contact chargingtechniques for charging the screen and other special expedients fornon-contact or field charging are used.

Modulated aperture electrostatic printing systems permitting directpositive printing without the necessity for contact charging the screenmodulator or other special expedients are described in other patentapplications assigned to the present assignee. In the U.S. Pat.application Ser. No. 85,070, now U.S. Pat. No. 3,694,200 entitledElectrostatic Modulator for Controlling Flow of Charged Particles, filedon Oct. 29, 1970, there is described an apertured screen havingsubstantially the entire surfaces formed of a dielectric material withthe dielectric preferably thicker on one face of the screen than on theother. The screen is charged with like charges over substantially itsentire surfaces to develop fringing fields in the apertures and ispreferably charged to a higher level on the thicker side so that thescreen initially carried a higher potential on one side providing auniform charge inequality or potential difference through the screenapertures. The charge inequality or potential difference is oriented toprovide enhancing fields within the apertures of the screen to enhancethe flow of charged particles passing through the screen from thedirection of the side of higher charge density. The charge distributedacross the side of the screen having a higher charge density isselectively dissipated in accordance with a pattern to be re producedthereby establishing a bi-polar electrostatic latent image as amodification of the fringing fields for density control of a flow ofcharged particles directed through the screen. Full modulation controlof the particle flow including blocking fields, enhancing fields, and aspectrum of fringing fields over a range in between, is thereforepossible with a single selective charge dissipation. This arrangementalso permits direct positive printing when the charge is selectivelydissipated by optical light image addressing. For optical addressing theinsulating material on the thicker side is a photoconductive insulatingmaterial. Thus, the passage of charged particles is enhanced orunimpeded in the dark areas while the passage of charged toner particlesis blocked or impeded in the light areas. A different modulated aperturenon-contact printing screen and system permitting direct positiveprinting is also set forth in the U.S. Pat. application Ser. No.197,877, entitled Method and Apparatus for Forming A PositiveElectrostatic Printing, filed on Nov. 1 1, 1971.

Another approach to modulated aperture noncontact electrostatic printingis set forth in U.S. Pat. application Ser. No. 864,022, now U.S. Pat.No. 3,713,734 entitled Electrostatic Line Printer, filed on Oct. 6, 1969and assigned to the assignee of the present case, now U. S. Pat. No.3,689,935. This arrangement is particularly applicable in electronicallyaddressed electrostatic printing and computer printout. TheElectrostatic Line Printer incorporates in one embodiment, amultilayered particle modulator comprising a layer of insulatingmaterial, a continuous layer of conducting material on one side of theinsulating layer, and a segmented layer of conducting material on theother side of the insulating layer. Both conductive layers can besegmented however. At least one row of apertures is formed through themultilayered particle modulator. Each segment of the segmented layer ofconductive material is formed adjacent or around at least one apertureand is insulatively isolated from every other segment of the segmentedconductive layer. More than one aperture may be formed through eachsegment. Selected potentials are applied to each of the segments of thesegmented conductive layer while a fixed potential is applied to thecontinuous conductive layer. Charged layers are thereby established oneither side of the insulating layer permitting fringing fields to beestablished within the modulator apertures oriented to block or enhancea flow of charged particles directed through the screen or to controlthe flow over a continuous range in between. The bi-polar fringingfields established within the apertures are separately controllableaccording to the electric potentials applied to the insulativelyisolated segments along the row of apertures. An overall applied fieldprojects charged particles through the row of apertures of the particlemodulator and the density of the particle stream is modulated accordingto the pattern of potentials applied to the segments of the segmentedconductive layer. A modulated stream of charged particles impinge upon aprint receiving medium interposed in the modulated particle stream andtranslated relative to the particle modulator to provide line by line orline scan printing.

In each of the above arrangements for modulated aperture non-contactelectrostatic printing the twodimensional screen or line grid modulatorcan be used to modulate a flow of charged toner particles or ions. Whenthe screen or line grid modulator is used to control a flow of tonerparticles either dry or liquid, a toner cloud is produced from a tonersource on the side of the modulator opposite the print receiving mediumand an overall applied accelerating field generates a stream of tonerparticles toward the print receiving medium which is modulated by theinterposed screen or line grid. This arrangement enables directdeposition of toner particles on any surface, regular or irregular,dielectric or conductive. However, a percentage of the toner particlescharged by the toner source on the other side of the screen or line griddo not pass through the apertures but land on solid portions of thescreen or line grid, or on walls of the toner supply ducts.

The use of ions in the particle flow, instead of toner marking materialitself, avoids any problem of toner buildup and further permits the useof lower potentials for gating the particle stream. However, the use ofan ion stream generated from a corona source for establishing anelectrostatic latent image on the print receiving medium requires theuse of high dielectric paper or treated paper in order to permitretention of sufficient charge magnitude in the electrostatic latentimage.

It is therefore an object of the present invention to provide amodulated aperture electrostatic printing system incorporating theadvantages of both modulated toner particle flow and modulated ionstream flow noncontact printing while suffering the disadvantages ofneither. In order to accomplish this result the present inventiongenerally contemplates the method of generating an ion stream in thedirection of a print receiving medium, modulating the cross sectionalflow density of ions in the stream in accordance with a pattern to bereproduced, and introducing a cloud of substantially uncharged tonermarking particles adjacent the print receiving medium whereby themodulated ion stream selectively impinges upon and charges tonerparticles in the cloud which are thereby deposited on the printreceiving medium in accordance with a pattern to be reproduced.

Thus, the invention generally contemplates providing in one embodiment acorona source and spaced electrode for generating a substantiallyuniform ion stream, and a support for positioning a print receivingmedium in the path of the ion stream. A multilayered aperturedtwo-dimensional screen or line grid modulator is interposed in the ionstream between the source and print receiving medium for modulating thecross sectional flow density of ions in the stream in accordance with apattern to be reproduced. Finally, a toner source is provided andarranged for introducing a cloud of substantially uncharged tonermarking particles adjacent the print receiving medium whereby themodulated ion stream selectively impinges upon and charges tonerparticles in the cloud. The selectively charged toner particles adjacentthe print receiving medium are thereafter accelerated and deposited onthe medium in accordance with the pattern to be reproduced.

A feature and advantage of this system is that charged toner particlesneed not be ducted or transported through channels in the machinery orthrough the screen or line grid modulator thereby avoiding toner lossand toner contamination while at the same time permitting direct tonerdeposition printing on the print receiving medium so that electrostaticprinting can take place on any surface.

Another feature and advantage of this arrangement is that the ion streamalone passes through the screen or line grid modulator permitting theuse of lower gating potentials, high frequency gate switching, and highvelocity particle transport for high speed printing.

In a preferred form of the invention, modulation of the ion flow isaccomplished using a multilayered apertured element spaced between theion source and the accelerating electrode having at least a conductivelayer and an insulative layer capable of supporting charged potentialsof differing magnitude on different layers of the element forestablishing the electrostatic lines of force within the apertures ofthe element for controlling passage of ions in accordance with a patternto be reproduced. Thus, the preferred embodiments generally include allof the two-dimensional screen and one-dimensional line grid modulatorsdescribed in the patent applications referred to above and assigned tothe assignee of the present case.

In order to further eliminate toner contamination in the printingmachinery the invention contemplates establishing an air pressuredifferential from one side of the modulating element to the other withthe low pressure on the side adjacent the print receiving medium wherebya small air flow is established through the apertures of the modulatingelement to prevent the toner particles from entering the apertures.According to another aspect of the invention, air streams can beinjected along the surfaces of the modulating element and positionedprint receiving medium in order to provide air boundaries around theintroduced toner cloud.

In the preferred forms, whether using the twodimensional screen or theone-dimensional line grid modulator, the invention contemplates line byline or line scan printing to provide optimum control over theuniformity of toner distribution. A feature and advantage of thisapproach is that toner cloud uniformity is necessary only along onemajor dimension namely the printing line. This result, when printingwith the twodimensional screen, is accomplished as follows.

A row corona source and spaced accelerating electrode generate asubstantially uniform stream of ions having a line or linear crosssectional configuration. A multilayered apertured screen formed with atwodimensional array of apertures for modulating the idn flow isinterposed in the space between the row corona source and theaccelerating electrode. The multilayered apertured screen is formed withat least a conductive layer and an insulative layer as heretoforedescribed capable of supporting charge potentials of different magnitudeon different layers of the screen for establishing electrostatic linesof force within the apertures of the screen for controlling passage ofions. Means are provided for translating the screen across the linearcross-section stream of ions for variably modulating the line stream inaccordance with the pattern of electrostatic lines of force within theapertures across the screen. At the same time a print receiving mediumis supported and positioned between the modulating screen and theaccelerating electrode in the path of the linear ion stream and theprint receiving medium is transported across the line cross-section ofthe ion stream at a rate synchronized with the modulating screen. it is,of course, the relative motion of the ion stream to the modulator andprint receiver which is useful, and thus, the latter two may be fixedand the ion stream translated. Finally a channel is provided forintroducing a cloud of substantially uncharged toner marking particlesinto the path of the linear crosssection ion stream inthe space betweenthe translating modulating screen and the translating print receivingmedium whereby the modulated linear cross-section ion stream selectivelyimpinges upon and charges toner particles in the cloud which areaccelerated and deposited on the print receiving medium in accordancewith the pattern to be reproduced. Preferably, a velocity component isimparted to the toner cloud substantially equal to and in the directionof motion of the modulating screen and print receiving medium.

The system also incorporates elements for establishing an electrostaticlatent image on the insulative layer of the two-dimensional modulatingscreen and for applying a potential to the conductive layer of thescreen. In one example optical addressing is utilized to establish theelectrostatic latent image and the insulative layer comprises a layer ofphotoconductive insulating material. The photoconductive layer is firstuniformly charged and the pattern to be reproduced is optically imagedonto the photoconductive layer for selectively dissipating the charge toform an electrostatic latent image. A potential is applied to theconductive layer to provide the bi-polar electrostatic latent image offringing fields within the apertures. The screen can be formed in theconfiguration of an elongate web transported along a plurality ofprinting stations for multiple copy electrostatic printing.

According to one embodiment of this aspect of the invention the screenis formed in the configuration of a hollow cylinder and a row coronasource is positioned inside the cylindrical screen with the acceleratingelectrode spaced from the row corona source outside the cylindricalscreen. The cylindrical screen is rotated relative to the row coronasource and the accelerating electrode to variably modulate the linecross-section ion flow in accordance with the pattern of electrostaticlines of force within the apertures across the screen. A printingreceiving medium is spaced from the cylindrical screen between thecylindrical screen modulator and accelerating electrodes and istransported across the path of the line cross-section ion flow at a ratesynchronized with the rotation of the cylinder. As in the previousembodiment a channel is provided for introducing a cloud ofsubstantially uncharged toner marking particles in the path of thelinear ion stream and the space between the cylindrical screen surfaceand the print receiving medium. The cylindrical screen may also beoptically addressed for establishing the double layer chargeelectrostatic latent image.

The invention is also applied in a novel method for erasure inelectrostatic printing with dry toner particles comprising depositing apattern of dry toner particles on a print receiving medium andselectively blowing the deposited dry toner particles off the printreceiving medium prior to fixing. Thus, an electrostatic printing headwith erasing capability is provided with a first channel arrangedbetween the modulating element and print receiving medium forintroducing a cloud of substantially uncharged dry toner markingparticles in the space between the modulating element and a positionedprint receiving medium. A modulated ion stream passing through themodulating elements selectively impinges upon and charges tonerparticles in the cloud which are accelerated and deposited upon theprint receiving medium in accordance with the pattern to be reproduced.The printing head, however, is also provided with a second channelarranged adjacent the surface of the positioned print receiving mediumfor delivering pulses of air against the print receiving medium at thelocation of dry toner particles desposited on the medium in order todisperse a pattern of toner particles deposited on the paper prior tofixing.

The invention thus generally contemplates the method of electrostaticprinting comprising generating an ion stream in the direction of a printreceiving medium, modulating the cross sectional density flow of ions inthe stream in accordance with a pattern to be reproduced, andintroducing a cloud of substantially uncharged toner marking particlesadjacent the print receiving medium whereby the modulated ion streamselectively impinges upon and charges toner particles in the cloud whichare deposited on the print receiving medium in accordance with a patternto be reproduced. in the event dry toner marking particles are used, theinvention also contemplates the additional step of selectively blowingthe so deposited toner particles off the print receiving medium forerasure.

The invention also contemplates a novel character sequential printingmethod using the electrostatic printing head according to which theprinting head is returned to its original position after the characterhas been printed by a small deflection of the paper carrier and return,a small deflection of the printing head and return, or electrostaticdeflection of the stream of charged particles.

Other objects, features and advantages of the present invention willbecome apparent in the following specification and accompanyingdrawings.

FIG. 1 is a fragmentary plan view and FIG. 1A a fragmentary sidecross-sectional view of a multilayered apertured screen for modulatingthe flow of charged particles.

FIG. 2 is a detailed cross-sectional view of one aperture of the screenshowing a portion of the double layer charge electrostatic latent imagesupported by the screen.

FIG. 3 is a fragmentary side cross-sectional view of anothermultilayered screen.

FIG. 4 is a sequence of diagrammatic views showing the steps inmodulated aperture electrostatic printing.

FIG. 5 is a fragmentary plan view and FIG. 5A a fragmentary sidecross-sectional view of a line or bar modulating element for modulatedaperture electrostatic line printing.

FIG. 6 is a diagrammatic view of a system for modulated apertureelectrostatic line printing.

FIG. 7 is a fragmentary side cross-sectional view of a system formodulated aperture electrostatic printing in which a flow of chargedtoner particles is modulated.

FIG. 8 is a fragmentary side cross-sectional view of a system formodulated aperture electrostatic printing according to the presentinvention in which a flow of ions is modulated to impinge on anintroduced cloud of toner marking particles.

FIG. 8A is a fragmentary perspective view of the system of FIG. 8.

FIGS. 9, 10 and 11 are fragmentary side crosssectional views ofvariations of the modulated aperture electrostatic printing systemillustrated in FIG. 8.

FIG. 12 is a diagrammatic side cross-sectional view of a system formodulated aperture electrostatic printing according to the presentinvention utilizing a twodimensional modulated aperture screen.

FIG. 13 is a diagrammatic view of another system for modulated apertureelectrostatic printing in which the screen is formed in theconfiguration of a drum or cylinder.

FIG. 14 is a diagrammatic side view of a system with multiple printingstations of the type shown in FIG. 13.

FIG. 15 is a fragmentary diagrammatic view of a system for opticallyaddressing the cylindrical screen to establish the double layer chargeelectrostatic latent image.

FIG. 16 is a diagrammatic side cross-sectional view of an electrostaticprinting head with erasing capability.

In order to more fully understand the invention, a general descriptionof modulated aperture electrostatic printing as heretofore developed isfirst presented with reference to FIGS. 1-7. In FIGS. 1 and 1A isillustrated a multilayered apertured screen suitable for use in thepresent invention. The screen 10 generally comprises a conductive layer11 and an insulative layer 12 with rows of holes 13 formed therethrough.As shown in more detail in FIG. 2, the construction of the screen issuch that a layer of charges can be supported on either side of theinsulative layer 12. Thus, a potential can be applied to the conductivelayer 11 in order to supply charges 14 of a particular sign such as, forexample, positive, along the side of the insulative layer 12 adjacentthe conductive layer 11. A second layer of charges 15 can be establishedalong the outer side of the insulative layer of 12 or for exampleopposite or negative charge around the apertures 13 of the screen. Thedou ble layer charge establishes lines of force within the apertures 13of the screen which can be oriented to either block or enhance a flow ofcharged particles directed through the apertures or to control thedensity of particles passing through the apertures over a continuousrange in between. The pattern of electrostatic lines of forceestablished within the apertures 13 and the screen is also referred toherein as a bi-polar electrostatic latent image.

The pattern of charges 15 on the outside of the insulative layer can beestablished by a variety of contact charging techniques or by opticaladdressing as set forth in the patent applications referred to above. Byway of example the insulative layer 12 can be formed of aphotoconductive insulating material. As shown in FIG. 4, the doublelayer screen 10 is first sprayed from a corona source 20, thephotoconductive insulative layer facing the corona source, and theconductive layer maintained at some fixed potential such as ground sothat a bi-polar electrostatic latent image is formed across theinsulative layer of the screen. The screen 10 is then exposed to lightfrom a source 21 in a pattern in accordance with an image to bereproduced thereby selectively to'dissipate the charge on thephotoconductive insulative layer. This step of course occurs in a darkenvironment. Finally, the screen 10 is positioned adjacent a printreceiving medium carried on an accelerating electrode 22 with a tonersource 23 positioned on the opposite side of the screen. A potentialderived from source 24 establishes an accelerating field between thetoner supply 23 and the accelerating electrode 22 so that a stream ofcharged toner particles is directed through the apertures of the screen10. At the same time, a selected potential from source 25 is applied tothe conductive layer of the screen to maintain the electrostatic fieldbetween the screen and source and between the screen and acceleratingelectrode. With blocking fields initially established across the screen,toner is permitted to pass through those apertures from which the chargeon the insulative layer is dissipated by light exposure in the previousstep, while the unexposed apertures continue to block the passage ofcharged toner particles forming a deposit of toner particles on theprint receiving medium at electrode 22 in accordance with the pattern tobe reproduced.

A variety of other modulating screen configurations are described in thepatent applications referred to above. By way of example in FIG. 3 thereis shown a three-layered apertured screen comprising a conductive layer26 and a photoconductive insulating layer 27 separated by ahigh'dielectric insulating material 28. A bi-polar or double layercharged electrostatic latent image is established across the insulativelayers of this screen according to a variety of charging techniques setforth in the patent applications referred to above.

Another type of modulating element for modulated aperture electrostaticline printing is set forth in FIGS. 5 and 5A. The modulating element 30consists of an elongate length or bar of insulating material 31 havingformed along one side a continuous layer of conducting material 32 andhaving formed along the opposite side a segmented conductive layer 33consisting of individual insulatively isolated segments 33A, 33B, 33C,etc.

The so formed elongate bar is formed with a row of aperturestherethrough, each aperture surrounded by a conductive segment 33A, 33B,33C etc. An electrical lead is provided to the continuous conductivelayer 32 for applying a uniform potential across one face of theinsulative layer 31 and around one side of each aperture. A plurality ofelectrical leads are provided one for each of the conductive segments33A, 33B, 33C, etc., -so that a different potential can be applied toeach of the segments for building up differing selective charge layersaround the other side of each of theapertures 35 in accordance with apattern to be reproduced. The double layer charge along each side of theinsulative layer 31 permits development of electrostatic lines of forceor fringing fields within the apertures for controlling the flow of alinear cross-sectional stream of ions directed through the modulatingelement. A system for electrostatic line printing using the line or barmodulator of FIGS. and 5A is shownin the diagrammatic view of FIG. 6. Atoner supply 40 and an accelerating electrode 41 are spaced apart andinterconnected with a potential source for charging and acceleratingtoner particles from the toner source 40 in the direction of theelectrode 41. The outlet from the toner source is shaped to provide anelongate or linear cross-sectional stream of toner particles. Themodulating element 30 is positioned in the path of the toner flow with abiasing potential applied to the conductive layer 32 via electrical lead41 and with selected electrical potentials ap plied to the segmentedelements to the segmented layer 33 via the bank of electrical leads 42which may be addressed for example by appropriate logic circuitry toprovide a changing pattern of charges across the segmented layer inaccordance with a pattern to be reproduced. The flow of toner particlesdirected through the apertures of the modulating element isappropriately shaped or modulated in accordance with the pattern to bereproduced and accelerated toward the back electrode 41. A printreceiving medium 43 is transported across the modulated toner stream ina space between the modulating element 30 and electrode 41 and the speedof transport of the paper or other print receiving medium 43 issynchronized with the frequency or rate of change of potentials appliedto the segmented portions of the segmented layer 33. As an alternativeto the continuous line printing described above, other modes ofmodulated aperture electrostatic line printing are set forth in thepatent application Ser. No. 864,022 now U.S. Pat. No. 3,689,935 referredto above.

Representative screens and systems for modulated aperture electrostaticprinting have been described in the foregoing discussion. Other examplesof modulated aperture printing screens and systems are set forth indetail in the patent applications referred to above. One example of aprinting operation of these systems is shown in FIG. 7 with reference toa line modulator of the type shown in FIGS. 5 and 5A. As illustrated inFIG. 7, a corona source 45, particle modulator 54, print receivingmedium 47, and accelerating electrode 48 are provided in sequence. Theaccelerating electrode 48 can itself form the support for the printreceiving medium 47. The corona source 45 generates a stream of chargedparticles 50 which are accelerated toward the back electrode 48 by asuitable potential difference applied between the corona source 45 andelectrode 48. Toner marking particles 51 are delivered by a source 52into the vicinity of the corona emissions and acquire charges. Thecharged toner particles 53 are thereafter accelerated toward theelectrode 48 through the apertures of the line modulator 46. Asheretofore described, the modulating element 46 consists of aninsulative layer 54 with conductive layers 55 and 56 formed on eitherside thereof, at least one of which is segmented into insulativelyisolated segments surrounding the apertures formed in a row along themodulating element. One or more rows of apertures can be used as setforth in US. Pat. application Ser. No. 864,022 referred to above.Potentials applied to the conductive layers over electrical lines 57 and58 one of which comprises a bank of electrical leads to the segments ofthe segmented layer selectively develop lines of force within theapertures in accordance with a pattern to be reproduced selectivelymodulating the flow of toner particles so that the particles come torest on the print receiving medium 47 in accordancewith the pattern tobe reproduced.

Alternatively, as described in theabove referenced patent application,the ion stream itself can be modulated by the modulating element so thatthe modulated stream of ions impinges upon the print receiving medium 47in accordance with the pattern to be reproduced. In this arrangementhigh dielectric paper, for example, is used for the print receivingmedium 47 in order to retain the electrostatic latent image deposited bythe ions. The paper is thereafter dusted and fixed in accordance withknown techniques of electrostatic printing.

According to the present invention, a novel system and method formodulated aperture electrostatic printing using screens of the type setforth above is provided. The basic system and method are depicted inFIGS. 8 and 8A in contrast to the arrangement of FIG. 7 withcorresponding elements numbered accordingly. Thus, according to theembodiment of the present invention illustrated in FIG. 8 there isprovided a corona source modulating element 46 print receiving medium 47and accelerating electrode 48 in sequence as described with reference toFIG. 7. Coronasource 45 generates a stream of ions which areaccelerating toward the back electrode 38 through the apertures ofmodulating element 46. In the example of FIG. 8A, a linear corona sourcein the form of a wire is shown, but a row of point source or othercorona source arrangement may be used. In contrast to the previouslydeveloped modulated aperture printing systems and methods describedabove, and in the patent applications referred to, the toner particles51 are introduced by a source 60 into the space between the modulatedelement 46 and positioned print receiving medium 47. The toner particles51 are neutral and form a cloud in the interspace between the modulatingelement 46 and print receiving medium 47. The ion stream passing throughthe apertures of the modulating element 46 and modulated according tothe array of fields established within the apertures, impinges on thecloud of uncharged toner particles selectively charging the particles inaccordance with the pattern to be reproduced so that the chargedparticles are accelerated by the electrode 48 to deposit on the printreceiving medium 47 in accordance with the pattern to be reproduced.This novel arrangement and method incorporates the advantages of boththe toner modulating and ion modulating systems described in the patentapplications referred to above. Thus, because only the ion stream ismodulated by the modulating element 46 lower potentials can be used onelectrical leads 57 and 58 with higher switching frequencies. Because ofthe higher transit speed of the ions printing can be conducted at higherspeeds. Toner loss at the screen or modulated element is avoided as areany cleaning problems. After the ions pass through the apertures of themodulating element they intercept and charge the toner particles whichare then propelled by the accelerating electrostatic field toward theelectrode to deposit on the print receiving medium 47. Thus, printingwith the toner particles is accomplished directly, rather than firstcreating an electrostatic latent image on the print receiving medium.Printing can therefore take place on any surface without the necessityof treated or high dielectric paper. Furthermore, because the injectedtoner particles 51 either in solid or aerosol form are neutral, there islittle tendency for toner to drift and deposit on the printing medium orother apparatus prior to charging by the ion stream. The toner particlesbecome charged toner particles 53 generally only upon impingement of orclose proximity to the ion stream 50.

A feature and advantage of this arrangement is that either liquidaerosol or solid particles can be used with equal facility. When thetoner stream itself is modulated with the modulating element or screen,the use of solid particles causes a percentage of particles to de positon the screen or modulating element thereby making cleaning stepsdesirable in the printing process. Therefore, when the toner streamitself is modulated liquid aerosols have been used to more easilyfacilitate removal of aerosol particles deposited on the printing screenor modulating element. Liquid droplets coalescing on the face of the baror screen modulator would be drawn off at the low edge by means of awick. In the configuration of the present invention as shown in FIGS. 8and 8A, however, toner is not deposited on the bar or screen modulatorand is kept out of the apertures. Therefore dry toner can be used withequal facility and fused by heat after printing is completed with thefuser located beyond the printing station.

In order to further prevent any drift of toner particles toward theprint receiving medium prior to charging and to further prevent anydrift of toner particles toward the modulating element or screen,protective air boundaries can be injected over the surface of themodulating element and paper as shown in FIG. 9. As shown in thatexample, the toner source 62 positioned for delivering toner in thespace between the modulating element 63 and print receiving medium 64 isshaped to define air passageways or channels 65 and 66 along the surfaceof the modulating element 63 and print receiving medium 64 respectively.Air under slightly higher pressure is delivered via channel 65 and 66 toeffectively contain the neutral toner particles within air boundariesuntil they become charged by impingement from the ion stream and areaccelerated toward electrode 67.

Another approach for preventing drift of uncharged toner particles intothe apertures of the modulating element is illustrated in FIG. 10.According to this arrangement a slight pressure differential isestablished on either side of the modulating element with the slightlyhigher pressure on the corona side. As a result, a very small air flow70 is established through the apertures of the modulating element 71thereby preventing the toner cloud from entering the apertures.

By means of the expedients illustrated in F lGS. 9 and 10, a directionalvelocity parallel to the plane of the printing surface can be impartedto the tomer cloud in a lateral direction between the print receivingmedium and modulating element. The velocity component imparted to thetoner cloud can be correlated with the speed of transport of the printreceiving medium in order to prevent any distortion in the reproducedpattern.

Another arrangement for delivering and picking up toner and imparting acontrolled lateral velocity component to the toner cloud is shown inFIG. 11. In that arrangement the toner source 75 tapers to a narrowdelivery outlet while a toner pick up channel 76 is positioned acrossfrom the toner source on the other side of the apertures of the screenor modulating element. The pick up channel 76 is provided with an inlethaving a larger diameter than the outlet from the toner source 75. Bymeans of differential pressures established within the channels 75 and76, the toner can be delivered with a controlled lateral component ofmotion synchronized with the velocity of transport of the printreceiving medium 77.

In one example of the electrostatic line printing embodiment of thepresent invention described above, charged toner particles wereaccelerated to a velocity of 15 centimeters per second through a linemodulating element having a thickness of 0.25 millimeters. The resultingtransit time of L6 milliseconds theoretically permits at highestaddressing frequency or switching frequency of the potentials applied tothe conductive layers of approximately 300 cycles per second. Apractical addressing frequency of approximately 100 cycles per secondwas achieved. The switching rate for controlling ions is greater by afactor of more than one hundred.

Embodiments of the present invention for modulated apertureelectrostatic printing using a screen formed with a two-dimensionalarray of apertures spaced between an ion source and acceleratingelectrode are illustrated in FlGS. 12 through 15. As showndiagrammatically in FIG. 12, the multilayered apertured screen 80 isinterposed between a corona source 81, and accelerating electrode 82having positioned across its surface a print receiving medium 83. Thescreen 80 is formed with a two-dimensional array of apertures and atleast a conductive layer and an insulative layer capable of supportingcharge potentials of differing magnitude on different layers of thescreen for establishing electrostatic lines of force within theapertures of the screen for controlling passage of ions in accordancewith a pattern to be reproduced as heretofore described. Uncharged tonerparticles 84 in the form of an aerosol or solid toner cloud isintroduced into the space between the screen 80 and the print receivingmedium 83. A stream of ions originating from corona source 81 in thedirection of accelerating electrode 82 intercepts the cloud of neutraltoner particles and the particles become charged and are transported inthe direction of the accelerating field. Because the ion flow ismodulated as it passes through the apparatus of screen 80 as heretoforedescribed, the toner cloud is selectively charged and the chargedparticles are attracted to the printing surface 83 by the acceleratingelectrostatic field in accordance with the pattern to be reproduced.Because the printing takes place over a twodimensional area, however,problems are encountered in the embodiment illustrated in FIG. 12 inuniformly distributing a cloud of toner particles 84 throughout thespace over the surface area to be printed. Nonuniform printing densitymay result.

In order to overcome this difficulty the invention is embodied in a formso that printing occurs at any instance only over a linear cross-sectionof the printing surface area even though a two-dimensional screen isutilized; This is accomplished according to one example as shown in FIG.13. In this arrangement, the printing or toning occurs along a narrowslit defined by baffles 90 while the screen 92 and print receivingmedium 93 are transported synchronously across the slit. The paper orother printing surface 93 is transported across the slit by means of adrum support 94, which also serves as the accelerating electrode, andappropriate rollers 95. A stream of ions generated by corona source 96is accelerated in the direction of the drum passing through theapertures of screen 92. Uncharged toner is delivered to the space overthe slit defined by baffles 90 in the path of the modulated ion streamthrough a channel defined by the baffie walls 90 and walls 91 so thatthe line cross-section ion stream passing through the apertures ofscreen 92 impinge upon the neutral cloud selectively charging particlesin the cloud which are accelerated toward the drum 94 to deposit on theprint receiving medium 93 in accordance with the pattern to bereproduced. The linear cross-section stream of ions originating from rowcorona source 96 is variably modulated as the screen 92 is transportedacross the ion stream path. The toner cloud is therefore variablycharged and comes to rest on the printing surface 93 in a continuouslychanging pattern as the paper or other printing surface is transportedacross the path of charged particles. Continuous line printing cantherefore be achieved.

In a preferred embodiment the cloud of toner particles 98 is deliveredthrough the channels defined by walls 90 and 91 with a lateral componentof motion in the direction of transport of the screen and printingsurface. By this expedient, distortion at the printing surface due todifferential velocity between the printing particles and printingsurface is avoided. Thus, the velocity component imparted to the cloudin the direction of transport is synchronized'with the velocity oftransport of the screen and paper or other printing surface.

The paper or other print receiving medium can be either sheet-fed orweb-fed. For simultaneous copies on web-fed paper, a system such as thatshown in FIG. 14 is provided. In this arrangement multiple printingstations 100, 101 and 102 of the type shown in FIG. 13 produceessentially simultaneous identical copies of the same charge imageformed across the two-dimensional multilayered apertured screen 103,except that the continuous line printing occurs simultaneously acrossdifferent linear cross-sectional portions of the screen 103. The amountof ion current used from the sources 104, 105 and 106 is adjusted to alevel so that the screen charge pattern is not discharged and thereforeso that three or more images can be printed from the same double layercharge bi-polar electrostatic latent image formed on the two-dimensionalscreen 103. The embodiment of FIG. 14 is particularly suited to computeroutput applications where web-fed paper is often used.

Another embodiment for the two-dimensional modulated aperture screensystem is shown in FIG. in

which the screen 110 is formed in the configuration of a drum 111. Inthis configuration the back electrode 112 which also serves as thecarrying element for the paper or other print receiving medium 13 iseither flat as illustrated in FIG. 15 or curved as shown in FIG. 13. Asin each of the previous embodiments, the substantially neutral tonercloud may consist either of solid or liquid toner particles introducedinto the channel defined by baffles 114 and walls 115. The toner isagain transported across the slit defined by the two sections of thechannel to be exposed to the linear cross-section ion stream originatingfrom the corona source 116 positioned inside the drum 111. The ionstream is modulated as it passes through the apertures of the screen110, which forms the surface of drum Ill. The toner cloud is selectivelycharged and accelerated toward the electrode 112 so that toner particlesdeposit on the print receiving medium 113 in the pattern to bereproduced as the paper or other medium 113 is transported across thepath of the charged particles. Continuous line printing is therebyachieved as the ion stream is variably modulated by the rotating drum.Again, a velocity component is preferably imparted to the toner cloud inthe direction of transport, synchronized with the velocity of thetransport of the drum surface and printing surface so that printingdistortions resulting from differential velocities are avoided.

Problems of printing smear are also eliminated by narrow slit printing.In this embodiment the width of the toner aperture slit formed by thetwo sides of the toner delivery channel formed by baffles 114 and walls115 has an effective width electrostatically as wide as a row of screenapertures formed in the twodimensional screen. Because the narrow slitis reduced in width to that of one aperture, the relative velocity ofpaper and screen and toner during printing is not critical in obtaininghigh resolutions. This permits mechanical simplicity to the design ofthe machine. However, the best performance is obtained when the screenvelocity, toner cloud velocity, and paper or other printing mediumvelocity are all nearly equal. The quantity of toner in the form ofliquid aerosol or solid particles in the space between the screen andprint receiving medium will then determine the density of the print fora given toner material, and the most uniform toning is then achieved.

In the embodiment of FIG. 15, the double layer charge bi-polarelectrostatic latent image is established across the screen by opticaladdressing in the manner heretofore described. In this example, thescreen is initially charged uniformly over the surface of aphotoconductive insulative layer by means of corona source 120. Thecharge on the photoconductive insulating layer is thereafter selectivelydissipated through the conductive layer which may be, for example,grounded. This is accomplished by imaging, by means of a lens system121, the light pattern to be reproduced, onto the surface of thephotoconductive insulating layer while the conductive layer is forexample grounded. A uniform potential is thereafter applied to theconductive layer so that selective fringing fields are establishedwithin the apertures of the screen for modulating the ion stream.

A feature and advantage of the modulated aperture electrostaticnon-contact printing system contemplated by the present invention isthat non-contact printing with dry toner particles is greatlyfacilitated as heretofore described. Thus, the invention enables anelectrostatic printing head with erasure capability for application in,for example, typewriters and consoles. According to this aspect, theinvention generally contemplates the electrostatic printing method offorming a pattern of dry toner particles on a print receiving medium inaccordance with a pattern to be reproduced and selectively blowing thedry toner particles off the print receiving medium prior to fixing.

A representative printing head for accomplishing this function is shownin FIG. 16. The printing head incorporates a modulating element 130 formodulated aperture electrostatic line printing of the type illustratedin FIG. 5 and 6. Thus, the modulating element 130 includes a centralinsulative layer with conductive layers coated on either side thereof atleast one of which consists of segments insulatively isolated andsurrounding apertures of a row of apertures formed across the modulatingelement for line printing. The conductive segments are electricallyaddressed for modulating a stream of ions in accordance with the lettersor other symbols to be reproduced by the printing head. The ion streamoriginates from corona source 131 in the direction of a backaccelerating electrode 132. A print receiving medium 133 is positionedin the path of ions originating from corona source 131 and passingthrough the apertures of screen 130. A first channel 134 is provideddefined by walls 135 and 136 for delivering or introducing a cloud ofsubstantially uncharged dry toner marking particles in the space betweenthe modulating element 130 and a positioned print receiving medium 133and adjacent the modulating element 130. The modulated ion streampassing through the apertures of the modulating element selectivelyimpinge upon and charge toner particles in the cloud which are 7 againstthe print receiving medium at the location of dry toner particlesdeposited on the medium in the pattern determined by the modulatingelement 130. Air pulses are initiated in channel 137 in order to erase apattern of toner particles erroneously deposited on the paper and priorto fixing.

From another point of view, the novel electrostatic typewriter printinghead consists of a first housing 138 having an opening for positioningadjacent a print re ceiving medium 133. A second housing 135 issupported within the first housing 138 so that the first and secondhousings 138 and 135 define a channel for delivering air pulses to thesurface of the print receiving medium through the opening. The secondhousing is also formed with an opening in alignment with the opening inthe first housing. A third housing 136 is positioned within the secondhousing 135, the second and third housing 135 and 136 defining a channel134 for introducing a cloud of substantially uncharged dry toner markingparticles in the space adjacent the opening in the second housing. Anapertured modulated grid element 130 for controlling the flow of ionsthrough the apertures of the modulating grid element is constructed withthe housing 136 so that the apertures are in substantial alignment withthe openings with the first and second housings. Finally, a coronadischarge source is positioned within the third housing 136 fordelivering a stream of ions in alignment with the apertures of themodulating element 130 and the openings within the first and secondhousings 138 and 135.

In a typewriter or console environment, the printing head is adapted forprinting individual symbols each in a continuous line printing mode andthereafter permitting erasure of any symbol erroneously deposited in theform of dry toner particles on the print receiving medium, by blowingoff and dispersing the deposited toner particles via the toner conduit134. The desired symbol can then be retoned or redeposited on theprinting surface in the same location.

This invention contemplates a variety of methods for this type ofcharacter sequential printing suitable for use in a continuous writingprinter such as typewriter or console. This type of operation isparticularly important when the printer is hand or keyboard addressedand where it is important for the operator to observe the printing as itis entered. To perform this function, where one character at a time isbeing printed, the writing head, as described with reference to FIG. 16,scans the distance of one character height, and during this scan formsthe addressed character in the appropriately indexed position. After thecharacter has been printed the position of the writing head is returnedto the original position, shifted one lateral position for printing thenext character. Or, the writing head is returned to the originalposition for erasure as heretofore described. The scanning action ofthis character sequential mode can be achieved in either of severalways. According to one method a small deflection of the paper carriageand return is used. According to a second method a small deflection ofthe printing head and return is used. And according to a third method,electrostatic deflection of the writing toner particles charged by theion stream accomplishes this scanning action. Indexing and characterforming and control operations are performed electronically with the useof shift registers.

What is claimed is:

1. Apparatus for electrostatic printing using marking particles impactedby ions propelled along a path comprising:

means for generating a substantially uniform ion stream;

means for supporting a print receiving medium in spaced relation to thegenerating means and in line with the path of the ion stream;

particle propelling electrical field means effective at least betweenthe means for generating and the print receiving medium for propellingions of the stream along the path toward the print receiving medium;

electrically controlled apertured means in the path between thegenerating means and the print receiving means for modulating thecross-sectional flow density of the ions in the stream along the path inaccordance with a pattern to be reproduced to convert the ion streaminto a modulated ion stream propelled toward the print receiving meansby said field means;

and means for introducing a cloud of marking particles, in which cloudthe particles carry as low a charge as possible, into the path of themodulated ion stream between the print receiving medium and theapertured means so that the modulated ion stream selectively impingesupon and charges marking particles in. the cloud and the field meanspropels the so-charged particles to the print receiving medium. I 2.Apparatus for electrostatic printing comprising: means for generatingand directing a substantially uniform ion stream along a path; aperturedmeans interposed in the ion stream for modulating the cross-sectionalflow density of ions in the stream along the path in accordance with apattern to be reproduced to convert the ion stream to a modulated ionstream directed along said path away from said modulating means;

means for supporting a print receiving medium in the path of themodulated ion stream;

said means for directing comprising electrical field means establishingan electrical field between the means for generating and the supportingmeans;

and means for introducing a cloud of marking particles into the path ofthe modulated ion stream between the print receiving medium and themeans for modulating whereby ions of the modulated ion streamselectively impinge upon and charge marking particles in the cloud whichare thereby directed by the electrical field toward, and deposited on,the print receiving medium in accordance with said pattern to bereproduced, said cloud of marking particles when so introduced being ina substantially uncharged state relative to the charge imparted to themby the ions.

3. Apparatus for electrostatic printing as set forth in claim 2, whereinthe modulating means comprises: a line printing modulator comprising alayer of insulating material, and layers of conductive material coatedon the sides respectively of the insulative layer, the modulator havingat least one row of apertures formed therethrough and at least one ofsaid conductive layers segmented into isolated segments each includingat least one aperture opening; said segments together with portions ofthe insulating layer covered thereby and the conductive layer on theside of the insulating layer opposite said opening supporting aplurality of separately controllable electric fields within saidapertures for modulating the stream of ions by selectively controllingthe passage of ions through the apertures, and means for applyingelectric potentials to each of said segments of the segmented layer ofconductive material to establish said separately controllable electricfields.

4. Apparatus for electrostatic printing as set forth in claim 2, whereinthe modulating'means comprises a multilayered apertured screen formedwith a twodimensional array of apertures therethrough, the screen havingat least a conductive layer and insulative layer overlaying theconductive layer, means for applying a potential to the conductive layerand means for establishing an electrostatic latent image on theinsulative layer for establishing electrostatic lines of force withinthe apertures across the screen for controlling passage of ions inaccordance with the electrostatic latent image.

5. A system for electrostatic printing comprising:

means for generating a directed stream of ions including an ion sourceand an electrode spaced from said ion source;

means interposed in the ion stream for modulating the ion flowcomprising a multilayered apertured modulating element spaced betweensaid ion source and said electrode having at least a conductive layerand an insulative layer capable of supporting charge potentials ofdiffering magnitude for establishing electrostatic lines of force withinthe apertures of the element for controlling passage of ions to form amodulated ion stream in accordance with a pattern to be reproduced;

means for supporting and positioning a print receiving medium spacedfrom said modulating element between said modulating element and saidelectrode;

means for establishing an electrical field between said ion source andsaid electrode encompassing said modulating element and the printreceiving medium;

and means for introducing a cloud of marking particles into the path ofthe modulated ion stream in the space between said modulating elementand the positioned print receiving medium whereby the modulated ionstream selectively impinges upon and charges marking particles in thecloud which are propelled by said electrical field and deposited on theprint receiving medium in accordance with the pattern to be reproduced;said marking particles when so-introduced being substantially unchargedrelative to the charge imparted to them by the ions.

6. Apparatus for electrostatic printing as set forth in claim 5, whereinsaid modulating means comprises a multilayered apertured screen formedwith a twodimensional array of apertures having at least said conductivelayer and insulative layer, means for applying a potential to saidconductive layer and means for establishing an electrostatic latentimage on said insulative layer for establishing electrostatic lines offorce within said apertures across the screen for controlling passage ofions therethrough in accordance with the electrostatic latent image; andmeans for establishing relative motion between said ion generating meansand said modulating element.

7. A system for electrostatic printing as set forth in claim 6, whereinthe multilayered apertured screen is formed in the configuration ofahollow cylinder and wherein said ion source ispositioned within thecylinder and said electrode is outside the cylinder.

8. A system for electrostatic printing as set forth in claim 5, furthercomprising means for establishing an air pressure differential from oneside of said modulating element to the other with the lower pressure onthe side adjacent the print receiving medium whereby a small air flow isestablished through the apertures of the modulating element to preventthe marking particles from entering the apertures of said modulatingelement.

9. A system for electrostatic printing as set forth in claim 5, furthercomprising means for projecting air streams along the surfaces of saidmodulating element and positioned print receiving medium to provide airboundaries around said introduced cloud.

10. Apparatus for electrostatic printing as set forth in claim 5,wherein said modulating means comprises a line printing modulatorcomprising a layer of insulating material, and layers of conductivematerial coated on the sides of the insulative layer respectively, saidmodulator having at least one row of apertures formed therethrough andat least one of said conductive layers segmented into isolated segments;and means for applying electric potentials to each of said segments ofthe segmented layer of conductive material for establishing a pluralityof separately controllable electric fields within said apertures formodulating a stream of ions passing through the aperatures.

11. A system for electrostatic printing as set forth in claim 5, furthercomprising means for imparting a lateral component of motion relative tothe modulated stream to the cloud substantially parallel to the plane ofthe print receiving medium where the print receiving medium receivescharged marking particles.

12. A method of electrostatic printing comprising:

generating an ion stream;

modulating the cross-sectional flow density of ions in the stream inaccordance with a pattern to be reproduced to produce a modulated streamof ions;

establishing an ion projection field for propelling the ions of the ionstream for modulation and propelling the ions of a modulated stream thusformed toward the print receiving medium;

and introducing a cloud of marking particles into the path of themodulated ion stream whereby the modulated ion stream selectivelyimpinges upon and charges marking particles in the cloud which aredeposited on the print receiving medium by said projection field inaccordance with a pattern to be reproduced; said marking particles whensointroduced being in as low a state of charge as possible.

13. A method of electrostatic printing as set forth in claim 12 furthercomprising the additional steps of translating the print receivingmedium; and imparting a velocity component to the cloud parallel to thesurface of the print receiving medium receiving the charged markingparticles and in the direction of translation thereof and substantiallysynchronized therewith.

14. A method for electrostatic printing comprising:

generating and propelling a stream of ions from an ion source toward anelectrode spaced from said ion source;

modulating the ion flow by interposing in the ion stream a multilayeredapertured grid having at least a conductive layer and an insulativelayer supporting charge potentials of differing magnitude on either sideof the insulative layer respectively establishing electrostatic lines offorce therebetween within the apertures to control passage of ionstherethrough and thereby to modulate the ion stream in accordance with apattern to be reproduced;

propelling the modulated ion stream exiting from the grid;

supporting and positioning a print receiving medium in the space betweenthe grid and electrode;

introducing a cloud of marking particles into the path of the modulatedion stream in the space between the grid and the positioned printreceiving medium whereby ions of the modulated ion stream selectivelyimpinge upon and charge marking particles in the cloud, said particlesso-introduced carrying substantially no charge relative to the chargeimparted to them by said ions;

and moving the so-charged particles in the direction of the printreceiving medium for deposit thereon in accordance with the pattern tobe reproduced.

15. A system for electrostatic printing as set forth in claim 14,further comprising means for establishing an electrostatic latent imageon the insulative layer of the modulating screen in accordance with thepattern in the form of electrostatic lines of force in the apertures;and means for applying a potential to the conductive layer.

16. A system for electrostatic printing comprising:

means for generating a stream of ions comprising row corona source meansand electrode means comprising an electrode spaced from said row coronasource means;

means interposed in the ion stream for modulating the ion streamcomprising a multilayered apertured screen formed with a two-dimensionalarray of apertures spaced between the ion corona source and theelectrode; said screen having at least a conductive layer and aninsulative layer capable of supporting charge potentials of differingmagnitude for establishing electrostatic lines of force within theapertures of the screen for controlling passage of ions in accordancewith a pattern to be reproduced;

means for translating said screen across the stream of ions formodulating the stream in accordance with the pattern of electrostaticlines of force within the apertures of the screen;

said electrode means moving the ions in the ion stream toward themodulating means and moving the ions exiting from the modulating meanstoward the electrode in the form of a modulated ion stream;

means for supporting and positioning a print receiving medium betweenthe modulating screen and the electrode in the path of the modulated ionstream;

means for transporting said print receiving medium across the modulatedion stream at substantially the same rate as the modulating screen istranslated across the stream of ions;

and channel means for introducing a cloud of marking particles in astate of charge as low as possible into the path of the modulated ionstream in the space between the modulating screen and the positionedprint receiving medium whereby the modulated ion stream selectivelyimpinges upon and charges marking particles in the cloud which aredeposited on the print receiving medium by the electrode means inaccordance with the pattern to be reproduced.

17. A system for electrostatic printing as set forth in claim 16,wherein the multilayered apertured screen is formed with the insulativelayer comprising a layer of photoconductive insulating material adjacentthe conductive layer, means for uniformly charging the photoconductivelayer, and means for optically imaging a pattern to be reproduced ontothe photoconductive layer for selectively dissipating the uniformcharging to form an electrostatic latent image of the pattern.

18. A system for electrostatic printing as set forth in claim 16,wherein said system has multiple copy capability, said screen formed inthe configuration of an elongate web; and a plurality of spaced apartprinting stations provided along the web; each printing stationcomprising a source of ions on one side of the web, a print receivingmedium on the other side of the web, means for translating each of saidlast mentioned print receiving mediums in synchronism with the web,means for introducing marking particles between the web and each of saidlast mentioned print receiving mediums, and means for propelling ionsfrom the last mentioned sources toward said last mentioned printreceiving mef diums selectively via the web for charging markingparticles by impact therewith whereby the so-charged marking particlesare deposited on the last mentioned print receiving mediums by thepropelling means to print multiple copies in accordance with saidpattern. 19. A system for electrostatic printing as set forth in claim16, further comprising means for imparting a component of motion to saidcloud in the direction of translation of the screen and transporting ofthe print receiving medium, said component of motion havingsubstantially the same velocity as said print receiving medium.

20. A system for electrostatic printing employing ions for chargingmarking particles comprising:

a screen for modulating a flow of ions comprising a multilayeredapertured screen having at least a conductive layer and an insulativelayer capable of supporting charge potentials of differing magnitude forestablishing electrostatic lines of force within the apertures of thescreen for controlling passage of ions in accordance with a pattern tobe reproduced, said screen formed in the configuration of a hollowcylinder; means for electrostatically charging the screen in accordancewith said pattern; means for generating a stream of ions, comprising arow corona source positioned inside the cylindrical screen and electrodemeans comprising an electrode spaced from said row corona source outsidethe cylindrical screen; electric field means for propelling chargedparticles including ions, said field means effective between the rowcorona source and the electrode; a means for rotating the cylindricalscreen through said .field means thereby to modulate the ion flow inaccordance with the pattern of electrostatic lines of forcewithinthe-apertures across the screen;

means for supporting and positioning a print receiving medium spacedfrom the screen between the screen and electrode; means for transportingsaid print receiving medium across the path of the modulated ion flowfrom the row corona source at a rate synchronized with the rotation ofthe cylindrical screen; and channel means for introducing a cloud ofmarking particles in substantially uncharged state relative to thecharge imparted to them by the ions in the path of the modulatedionstream in the space between the screen surface and the positionedprint receiving medium whereby the modulated ion stream selectivelyimpinges upon and charges particles in the cloud; said field meanspropelling the so-charged particles to the print receiving medium inaccordance with the pattern to be reproduced. .21. A system forelectrostatic printing as set forth in claim 20, wherein the insulativelayer of the modulating screen comprises a photoconductive layer on theouter surface of the cylindrical screen; and, wherein the means forcharging the screen establishes substantially uniform charge on thephotoconductive layer and optically images the pattern to be reproducedon the photoconductive layer for selectively dissipating the charge andestablishing an electrostatic latent image, and further comprising meansfor applying a potential to the conductive layer of said screen.

22. A system for electrostatic printing as set forth in claim 20,wherein said means for supporting and positioning a print receivingmedium comprises a drum.

23. An electrostatic printing head employing ions for charging markingmaterial particles comprising:

a multilayered apertured modulating grid element having at least aconductive layer and an insulative layer capable of supporting chargepotentials of differing magnitude for establishing electrostatic linesof force within the apertures of the element for controlling passage ofions through the apertures in accordance with a pattern to bereproduced;

means for generating and propelling a stream of ions comprising a coronasource positioned on one side of the modulating grid element andelectrode means spaced from the modulating grid element on the otherside thereof;

means for supporting and positioning a print receiving medium spacedfrom the modulating element between the modulating element and electrodemeans;

first channel means arranged between the modulating element and printreceiving medium adjacent the modulating element for introducing a cloudof dry toner marking particles in substantially uncharged state relativeto the charge imparted to the particles by the ions in the space betweenthe modulating element and the positioned print receiving medium wherebythe modulated ion stream passing through the modulating elementselectively impinges upon and charges toner particles in the cloud whichare thereby moved by the propelling means and deposited on the printreceiving medium in accordance with the pattern to be reproduced;

second channel means arranged between the modulating element and printreceiving medium adjacent the surface of the positioned print receivingmedium for delivering pulsed air against the print receiving medium atthe location of dry toner particles deposited on the medium in thepattern determined by said modulating element;

and means for initiating an air pulse in said second channel means inorder to erase a pattern of toner deposited on the paper prior tofixing.

24. A system for electrostatic printing as set forth in claim 23,wherein the modulating grid element comprises a line printing modulatorcomprising a layer of insulating material, and layers of conductivematerial coated on each side of the insulative layer, said multilayeredmodulator having at least one row of apertures formed therethrough andat least one conductive layer segmented into isolated segments forestablishing a plurality of separately controllable electric fieldsrespectively within said apertures for modulating the stream of ionsdirected toward the grid element, and means for applying electricpotentials to each of said segments of the segmented layer of conductivematerial.

25. An ion flow electrostatic typewriter printing head comprising:

a first housing having an opening for positioning adjacent a printreceiving medium;

dium in accordance with a pattern to be reproduced; and blowing the sodeposited pattern of toner particles off the print receiving medium toachieve erasm 28. A method of electrostatically controlled printingemploying ions for charging marking material for deposition on a printreceiving medium comprising the steps of:

generating the ions;

second housing means supported within said first housing means, saidfirst and second housing means defining a channel for delivering airpulses to the surface of the print receiving medium through saidopening, said second housing means also formed with an opening insubstantial align ment with the opening in said first housing means;third housing means positioned within said second housing means, saidsecond and third housing means for introducing a cloud of dry tonermarking particles carrying substantially no charge relative to thecharge imparted to them by the ions into the path of the modulated ionstream in the space between the modulating grid element and the printreceiving medium whereby the modulated ion stream selectively impingesupon and charges toner particles in the cloud; said means for propellingcharged particles propelling the so-charged toner particles to the printreceiving medium in accordance with the selected pattern to bereproduced;

and means for delivering pulses of air to the surface of the printreceiving medium at the location of toner particles, deposited on theprint receiving medium in a pattern determined by the modulating grid,thereby to permit dispersal of the deposited toner pattern prior tofixing.

27. A method of erasable electrostatic printing employing ions forcharging marking particles comprising:

generating an ion stream in the direction of a print receiving medium;

modulating the cross-sectional density flow of ions in the stream inaccordance with a pattern to be reproduced;

means defining a channel for introducing a cloud l0 of dry toner markingparticles in which the partil Stream of Ions; cles carry as low chargeas possible in the space inmterposmg apertured modlllator means m pathof Side the opening in the Second housing means; the stream to defineportions ofa plurality of elecan apertured modulating grid element forcontrolling y Selectable for il g T Ions to flow of ions through theapertures of the modulatt e p? receiving me i m0 u ator d withincomprising at least a conductive layer overlay ng mg elemenF saidelement posmone an insulative layer with the layers having coincldmgSaid thud bousmg mffans that h apertures apertures defining saidportions of the paths; lhereof are Substantial ahgnment the open atleast partially opening and closing said paths at the mgs from i firstand Seconc! l modulator means in accordance with a pattern to and acorona discharge sourcewithm said thlrd housbe reproduced on the printreceiving medium;

mg means ubstamlally m alfgnment introducing a cloud of marking materialparticles into l of modulalmg 5 and the opemngs said paths between themodulator means and the Sald first and second hollsmg m f printreceiving medium, said particles soelectrostatic yp l' Prlmmg head forerasintroduced carrying substantially no charge relaable electrostaticprinting on a print receiving medium tive to the charge imparted to themby Said ions; using ions for g g y toner marking Particles propellingthe ions along said paths selectively via the comprising: modulatormeans into the cloud for charging markmeans for generating ions; meansfor p p g ing material impacted by the ions and propelling chargedparticles including said ions in a stream the so-charged markingmaterial directly to the toward the print receiving medium; printreceiving medium; and an electrically modulatable grid elementinterposed establishing relative movement of the paths to the in the ionstream for modulating the cross-sectional print receiving medium insynchronization with the flow density of ions in the stream inaccordance opening and closing of said paths at the modulator with aselected typewriting pattern for exiting a means in accordance with saidpattern. d l d Stream f i 29. An electrostatically controlled printingprocess employing ions for charging marking material for deposition on aprint receiving medium, comprising in combination;

a source of ions for generating an ion stream; apertured modulator meansdisposed in the path of the stream to define portions of a plurality ofelectrically selectable paths between the source and the print receivingmedium; said modulator means comprising at least a conductive layeroverlaying an insulative layer with the layers having coincidingapertures to comprise said portions of the selectable paths; means forselectively at least partially opening and closing said paths at themodulator means in accordance with a pattern to be reproduced on theprint receiving medium; means for introducing a cloud of markingmaterial particles into the space between the modulator means and theprint receiving medium, said particles so-introduced carryingsubstantially no charge relative to the charge imparted to them by saidions; means for propelling ions along said selectable paths andselectively via the modulator means into the cloud for charging markingmaterial impacted and introducing a Cloud Of toner marking particles infor propelling the 0.charged marking material which the particles carrysubstantially no charge rectly to the print receiving medium; and,

relati e to the charge imparted to them y the ions means forestablishing relative movement of the seinto the path of the modulatedion stream adjacent lectable paths to the print receiving medium insynthe print receiving medium whereby the modulated chronization withthe opening and closing of said ion stream selectively impinges upon andcharges toner particles in the cloud; depositing the socharged tonerparticles on the print receiving mepaths by the means for selectivelyopening and closing the paths in accordance with said pattern.

1. Apparatus for electrostatic printing using marking particles impactedby ions propelled along a path comprising: means for generating asubstantially uniform ion stream; means for supporting a print receivingmedium in spaced relation to the generating means and in line with thepath of the ion stream; particle propelling electrical field meanseffective at least between the means for generating and the printreceiving medium for propelling ions of the stream along the path towardthe print receiving medium; electrically controlled apertured means inthe path between the generating means and the print receiving means formodulating the cross-sectional flow density of the ions in the streamalong the path in accordance with a pattern to be reproduced to convertthe ion stream into a modulated ion stream propelled toward the printreceiving means by said field means; and means for introducing a cloudof marking particles, in which cloud the particles carry as low a chargeas possible, into the path of the modulated ion stream between the printreceiving medium and the apertured means so that the modulated ionstream selectively impinges upon and charges marking particles in thecloud and the field means propels the so-charged particles to the printreceiving medium.
 2. Apparatus for electrostatic printing comprising:means for generating and directing a substantially uniform ion streamalong a path; apertured means interposed in the ion stream formodulating the cross-sectional flow density of ions in the stream alongthe path in accordance with a pattern to be reproduced to convert theion stream to a modulated ion stream directed along said path away fromsaid modulating means; means for supporting a print receiving medium inthe path of the modulated ion stream; said means for directingcomprising electrical field means establishing an electrical fieldbetween the means for generating and the supporting means; and means forintroducing a cloud of marking particles into the path of the modulatedion stream between the print receiving medium and the means formodulating whereby ions of the modulated ion stream selectively impingeupon and charge marking particles in the cloud which are therebydirected by the electrical field toward, and deposited on, the printreceiving medium in accordance with said pattern to be reproduced, saidcloud of marking particles when so introduced being in a substantiallyuncharged state relative to the charge imparted to them by the ions. 3.Apparatus for electrostatic printing as set forth in claim 2, whereinthe modulating means comprises: a line printing modulator comprising alayer of insulating material, and layers of conductive material coatedon the sides respectively of the insulative layer, the modulator havingat least one row of apertures formed therethrough and at least one ofsaid conductive layers segmented into isolated segments each includingat least one aperture opening; said segments together with portions ofthe insulating laYer covered thereby and the conductive layer on theside of the insulating layer opposite said opening supporting aplurality of separately controllable electric fields within saidapertures for modulating the stream of ions by selectively controllingthe passage of ions through the apertures, and means for applyingelectric potentials to each of said segments of the segmented layer ofconductive material to establish said separately controllable electricfields.
 4. Apparatus for electrostatic printing as set forth in claim 2,wherein the modulating means comprises a multilayered apertured screenformed with a two-dimensional array of apertures therethrough, thescreen having at least a conductive layer and insulative layeroverlaying the conductive layer, means for applying a potential to theconductive layer and means for establishing an electrostatic latentimage on the insulative layer for establishing electrostatic lines offorce within the apertures across the screen for controlling passage ofions in accordance with the electrostatic latent image.
 5. A system forelectrostatic printing comprising: means for generating a directedstream of ions including an ion source and an electrode spaced from saidion source; means interposed in the ion stream for modulating the ionflow comprising a multilayered apertured modulating element spacedbetween said ion source and said electrode having at least a conductivelayer and an insulative layer capable of supporting charge potentials ofdiffering magnitude for establishing electrostatic lines of force withinthe apertures of the element for controlling passage of ions to form amodulated ion stream in accordance with a pattern to be reproduced;means for supporting and positioning a print receiving medium spacedfrom said modulating element between said modulating element and saidelectrode; means for establishing an electrical field between said ionsource and said electrode encompassing said modulating element and theprint receiving medium; and means for introducing a cloud of markingparticles into the path of the modulated ion stream in the space betweensaid modulating element and the positioned print receiving mediumwhereby the modulated ion stream selectively impinges upon and chargesmarking particles in the cloud which are propelled by said electricalfield and deposited on the print receiving medium in accordance with thepattern to be reproduced; said marking particles when so-introducedbeing substantially uncharged relative to the charge imparted to them bythe ions.
 6. Apparatus for electrostatic printing as set forth in claim5, wherein said modulating means comprises a multilayered aperturedscreen formed with a two-dimensional array of apertures having at leastsaid conductive layer and insulative layer, means for applying apotential to said conductive layer and means for establishing anelectrostatic latent image on said insulative layer for establishingelectrostatic lines of force within said apertures across the screen forcontrolling passage of ions therethrough in accordance with theelectrostatic latent image; and means for establishing relative motionbetween said ion generating means and said modulating element.
 7. Asystem for electrostatic printing as set forth in claim 6, wherein themultilayered apertured screen is formed in the configuration of a hollowcylinder and wherein said ion source is positioned within the cylinderand said electrode is outside the cylinder.
 8. A system forelectrostatic printing as set forth in claim 5, further comprising meansfor establishing an air pressure differential from one side of saidmodulating element to the other with the lower pressure on the sideadjacent the print receiving medium whereby a small air flow isestablished through the apertures of the modulating element to preventthe marking particles from entering the apertures of said modulatingelement.
 9. A system for electrostatic printing as set forth in clAim 5,further comprising means for projecting air streams along the surfacesof said modulating element and positioned print receiving medium toprovide air boundaries around said introduced cloud.
 10. Apparatus forelectrostatic printing as set forth in claim 5, wherein said modulatingmeans comprises a line printing modulator comprising a layer ofinsulating material, and layers of conductive material coated on thesides of the insulative layer respectively, said modulator having atleast one row of apertures formed therethrough and at least one of saidconductive layers segmented into isolated segments; and means forapplying electric potentials to each of said segments of the segmentedlayer of conductive material for establishing a plurality of separatelycontrollable electric fields within said apertures for modulating astream of ions passing through the aperatures.
 11. A system forelectrostatic printing as set forth in claim 5, further comprising meansfor imparting a lateral component of motion relative to the modulatedstream to the cloud substantially parallel to the plane of the printreceiving medium where the print receiving medium receives chargedmarking particles.
 12. A method of electrostatic printing comprising:generating an ion stream; modulating the cross-sectional flow density ofions in the stream in accordance with a pattern to be reproduced toproduce a modulated stream of ions; establishing an ion projection fieldfor propelling the ions of the ion stream for modulation and propellingthe ions of a modulated stream thus formed toward the print receivingmedium; and introducing a cloud of marking particles into the path ofthe modulated ion stream whereby the modulated ion stream selectivelyimpinges upon and charges marking particles in the cloud which aredeposited on the print receiving medium by said projection field inaccordance with a pattern to be reproduced; said marking particles whenso-introduced being in as low a state of charge as possible.
 13. Amethod of electrostatic printing as set forth in claim 12 furthercomprising the additional steps of translating the print receivingmedium; and imparting a velocity component to the cloud parallel to thesurface of the print receiving medium receiving the charged markingparticles and in the direction of translation thereof and substantiallysynchronized therewith.
 14. A method for electrostatic printingcomprising: generating and propelling a stream of ions from an ionsource toward an electrode spaced from said ion source; modulating theion flow by interposing in the ion stream a multilayered apertured gridhaving at least a conductive layer and an insulative layer supportingcharge potentials of differing magnitude on either side of theinsulative layer respectively establishing electrostatic lines of forcetherebetween within the apertures to control passage of ionstherethrough and thereby to modulate the ion stream in accordance with apattern to be reproduced; propelling the modulated ion stream exitingfrom the grid; supporting and positioning a print receiving medium inthe space between the grid and electrode; introducing a cloud of markingparticles into the path of the modulated ion stream in the space betweenthe grid and the positioned print receiving medium whereby ions of themodulated ion stream selectively impinge upon and charge markingparticles in the cloud, said particles so-introduced carryingsubstantially no charge relative to the charge imparted to them by saidions; and moving the so-charged particles in the direction of the printreceiving medium for deposit thereon in accordance with the pattern tobe reproduced.
 15. A system for electrostatic printing as set forth inclaim 14, further comprising means for establishing an electrostaticlatent image on the insulative layer of the modulating screen inaccordance with the pattern in the form of electrostatic lines of forcein the apertures; and means for applying a potential to the conductivelayer.
 16. A system for electrostatic printing comprising: means forgenerating a stream of ions comprising row corona source means andelectrode means comprising an electrode spaced from said row coronasource means; means interposed in the ion stream for modulating the ionstream comprising a multilayered apertured screen formed with atwo-dimensional array of apertures spaced between the ion corona sourceand the electrode; said screen having at least a conductive layer and aninsulative layer capable of supporting charge potentials of differingmagnitude for establishing electrostatic lines of force within theapertures of the screen for controlling passage of ions in accordancewith a pattern to be reproduced; means for translating said screenacross the stream of ions for modulating the stream in accordance withthe pattern of electrostatic lines of force within the apertures of thescreen; said electrode means moving the ions in the ion stream towardthe modulating means and moving the ions exiting from the modulatingmeans toward the electrode in the form of a modulated ion stream; meansfor supporting and positioning a print receiving medium between themodulating screen and the electrode in the path of the modulated ionstream; means for transporting said print receiving medium across themodulated ion stream at substantially the same rate as the modulatingscreen is translated across the stream of ions; and channel means forintroducing a cloud of marking particles in a state of charge as low aspossible into the path of the modulated ion stream in the space betweenthe modulating screen and the positioned print receiving medium wherebythe modulated ion stream selectively impinges upon and charges markingparticles in the cloud which are deposited on the print receiving mediumby the electrode means in accordance with the pattern to be reproduced.17. A system for electrostatic printing as set forth in claim 16,wherein the multilayered apertured screen is formed with the insulativelayer comprising a layer of photoconductive insulating material adjacentthe conductive layer, means for uniformly charging the photoconductivelayer, and means for optically imaging a pattern to be reproduced ontothe photoconductive layer for selectively dissipating the uniformcharging to form an electrostatic latent image of the pattern.
 18. Asystem for electrostatic printing as set forth in claim 16, wherein saidsystem has multiple copy capability, said screen formed in theconfiguration of an elongate web; and a plurality of spaced apartprinting stations provided along the web; each printing stationcomprising a source of ions on one side of the web, a print receivingmedium on the other side of the web, means for translating each of saidlast mentioned print receiving mediums in synchronism with the web,means for introducing marking particles between the web and each of saidlast mentioned print receiving mediums, and means for propelling ionsfrom the last mentioned sources toward said last mentioned printreceiving mediums selectively via the web for charging marking particlesby impact therewith whereby the so-charged marking particles aredeposited on the last mentioned print receiving mediums by thepropelling means to print multiple copies in accordance with saidpattern.
 19. A system for electrostatic printing as set forth in claim16, further comprising means for imparting a component of motion to saidcloud in the direction of translation of the screen and transporting ofthe print receiving medium, said component of motion havingsubstantially the same velocity as said print receiving medium.
 20. Asystem for electrostatic printing employing ions for charging markingparticles comprising: a screen for modulating a flow of ions comprisinga multilayered apertured screen having at least a conductive layer andan insulative layer capable of supporting cHarge potentials of differingmagnitude for establishing electrostatic lines of force within theapertures of the screen for controlling passage of ions in accordancewith a pattern to be reproduced, said screen formed in the configurationof a hollow cylinder; means for electrostatically charging the screen inaccordance with said pattern; means for generating a stream of ions,comprising a row corona source positioned inside the cylindrical screenand electrode means comprising an electrode spaced from said row coronasource outside the cylindrical screen; electric field means forpropelling charged particles including ions, said field means effectivebetween the row corona source and the electrode; means for rotating thecylindrical screen through said field means thereby to modulate the ionflow in accordance with the pattern of electrostatic lines of forcewithin the apertures across the screen; means for supporting andpositioning a print receiving medium spaced from the screen between thescreen and electrode; means for transporting said print receiving mediumacross the path of the modulated ion flow from the row corona source ata rate synchronized with the rotation of the cylindrical screen; andchannel means for introducing a cloud of marking particles insubstantially uncharged state relative to the charge imparted to them bythe ions in the path of the modulated ion stream in the space betweenthe screen surface and the positioned print receiving medium whereby themodulated ion stream selectively impinges upon and charges particles inthe cloud; said field means propelling the so-charged particles to theprint receiving medium in accordance with the pattern to be reproduced.21. A system for electrostatic printing as set forth in claim 20,wherein the insulative layer of the modulating screen comprises aphotoconductive layer on the outer surface of the cylindrical screen;and, wherein the means for charging the screen establishes substantiallyuniform charge on the photoconductive layer and optically images thepattern to be reproduced on the photoconductive layer for selectivelydissipating the charge and establishing an electrostatic latent image,and further comprising means for applying a potential to the conductivelayer of said screen.
 22. A system for electrostatic printing as setforth in claim 20, wherein said means for supporting and positioning aprint receiving medium comprises a drum.
 23. An electrostatic printinghead employing ions for charging marking material particles comprising:a multilayered apertured modulating grid element having at least aconductive layer and an insulative layer capable of supporting chargepotentials of differing magnitude for establishing electrostatic linesof force within the apertures of the element for controlling passage ofions through the apertures in accordance with a pattern to bereproduced; means for generating and propelling a stream of ionscomprising a corona source positioned on one side of the modulating gridelement and electrode means spaced from the modulating grid element onthe other side thereof; means for supporting and positioning a printreceiving medium spaced from the modulating element between themodulating element and electrode means; first channel means arrangedbetween the modulating element and print receiving medium adjacent themodulating element for introducing a cloud of dry toner markingparticles in substantially uncharged state relative to the chargeimparted to the particles by the ions in the space between themodulating element and the positioned print receiving medium whereby themodulated ion stream passing through the modulating element selectivelyimpinges upon and charges toner particles in the cloud which are therebymoved by the propelling means and deposited on the print receivingmedium in accordance with the pattern to be reproduced; second channelmeans arranged between the modulating element and print receiving mediumadjacent the surface of the positioned print receiving medium fordelivering pulsed air against the print receiving medium at the locationof dry toner particles deposited on the medium in the pattern determinedby said modulating element; and means for initiating an air pulse insaid second channel means in order to erase a pattern of toner depositedon the paper prior to fixing.
 24. A system for electrostatic printing asset forth in claim 23, wherein the modulating grid element comprises aline printing modulator comprising a layer of insulating material, andlayers of conductive material coated on each side of the insulativelayer, said multilayered modulator having at least one row of aperturesformed therethrough and at least one conductive layer segmented intoisolated segments for establishing a plurality of separatelycontrollable electric fields respectively within said apertures formodulating the stream of ions directed toward the grid element, andmeans for applying electric potentials to each of said segments of thesegmented layer of conductive material.
 25. An ion flow electrostatictypewriter printing head comprising: a first housing having an openingfor positioning adjacent a print receiving medium; second housing meanssupported within said first housing means, said first and second housingmeans defining a channel for delivering air pulses to the surface of theprint receiving medium through said opening, said second housing meansalso formed with an opening in substantial alignment with the opening insaid first housing means; third housing means positioned within saidsecond housing means, said second and third housing means defining achannel for introducing a cloud of dry toner marking particles in whichthe particles carry as low charge as possible in the space inside theopening in the second housing means; an apertured modulating gridelement for controlling flow of ions through the apertures of themodulating grid element, said element positioned within said thirdhousing means so that the apertures thereof are in substantial alignmentwith the openings from said first and second housing means; and a coronadischarge source within said third housing means substantially inalignment with the apertures of said modulating grid and the openings insaid first and second housing means.
 26. An electrostatic typewriterprinting head for erasable electrostatic printing on a print receivingmedium using ions for charging dry toner marking particles comprising:means for generating ions; means for propelling charged particlesincluding said ions in a stream toward the print receiving medium; anelectrically modulatable grid element interposed in the ion stream formodulating the cross-sectional flow density of ions in the stream inaccordance with a selected typewriting pattern for exiting a modulatedstream of ions; means for introducing a cloud of dry toner markingparticles carrying substantially no charge relative to the chargeimparted to them by the ions into the path of the modulated ion streamin the space between the modulating grid element and the print receivingmedium whereby the modulated ion stream selectively impinges upon andcharges toner particles in the cloud; said means for propelling chargedparticles propelling the so-charged toner particles to the printreceiving medium in accordance with the selected pattern to bereproduced; and means for delivering pulses of air to the surface of theprint receiving medium at the location of toner particles, deposited onthe print receiving medium in a pattern determined by the modulatinggrid, thereby to permit dispersal of the deposited toner pattern priorto fixing.
 27. A method of erasable electrostatic printing employingions for charging marking particles comprising: generating an ion streamin the direction of a print receiving medium; modulating thecross-sectional density flow of ions in the strEam in accordance with apattern to be reproduced; introducing a cloud of toner marking particlesin which the particles carry substantially no charge relative to thecharge imparted to them by the ions into the path of the modulated ionstream adjacent the print receiving medium whereby the modulated ionstream selectively impinges upon and charges toner particles in thecloud; depositing the so-charged toner particles on the print receivingmedium in accordance with a pattern to be reproduced; and blowing the sodeposited pattern of toner particles off the print receiving medium toachieve erasing.
 28. A method of electrostatically controlled printingemploying ions for charging marking material for deposition on a printreceiving medium comprising the steps of: generating the ions; forming astream of ions; interposing apertured modulator means in the path of thestream to define portions of a plurality of electrically selectablepaths for the generated ions to the print receiving medium, saidmodulator means comprising at least a conductive layer overlaying aninsulative layer with the layers having coinciding apertures definingsaid portions of the paths; at least partially opening and closing saidpaths at the modulator means in accordance with a pattern to bereproduced on the print receiving medium; introducing a cloud of markingmaterial particles into said paths between the modulator means and theprint receiving medium, said particles so-introduced carryingsubstantially no charge relative to the charge imparted to them by saidions; propelling the ions along said paths selectively via the modulatormeans into the cloud for charging marking material impacted by the ionsand propelling the so-charged marking material directly to the printreceiving medium; and establishing relative movement of the paths to theprint receiving medium in synchronization with the opening and closingof said paths at the modulator means in accordance with said pattern.29. An electrostatically controlled printing process employing ions forcharging marking material for deposition on a print receiving medium,comprising in combination; a source of ions for generating an ionstream; apertured modulator means disposed in the path of the stream todefine portions of a plurality of electrically selectable paths betweenthe source and the print receiving medium; said modulator meanscomprising at least a conductive layer overlaying an insulative layerwith the layers having coinciding apertures to comprise said portions ofthe selectable paths; means for selectively at least partially openingand closing said paths at the modulator means in accordance with apattern to be reproduced on the print receiving medium; means forintroducing a cloud of marking material particles into the space betweenthe modulator means and the print receiving medium, said particlesso-introduced carrying substantially no charge relative to the chargeimparted to them by said ions; means for propelling ions along saidselectable paths and selectively via the modulator means into the cloudfor charging marking material impacted and for propelling the so-chargedmarking material directly to the print receiving medium; and, means forestablishing relative movement of the selectable paths to the printreceiving medium in synchronization with the opening and closing of saidpaths by the means for selectively opening and closing the paths inaccordance with said pattern.